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Hello Kent, thanks for participating in the CQD, and well this seems like a straightforward question, it has more to it. Article 334 of the NEC® provides guidance on the installation of NM Cable. Section 334.80 provides additional rules for the ampacity of NM Cable. In general, the ampacity of NM Cable cannot exceed that of a 60° rated conductor. Taking this rule to the 60° column of Table 310.16, the maximum ampactiy for #8 AWG copper conductor in the NM Cable would be 40A. Applying the same process for a #6 AWG copper conductor in the NM Cable, the maximum ampacity would be 55A.
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